Recent years have seen rapid promotion and application of high-voltage high-power converters in industrial manufacturing and transportation, thanks to their excellent properties and energy saving effectiveness. On the other hand, voltage withstand capacity of switching devices has severely constrained development of high voltage frequency conversion techniques. To obtain higher output voltage on the basis of current level of switching devices, multi-level inverters find wide applications in industrial manufacturing, transportation, and aerospace, owing to their high quality of output power, low voltage stress, and low switching loss. Topologies of a multi-level inverter mainly include diode-clamped, flying capacitor, or cascaded multi-level inverters. Among them, a bridge multi-level inverter finds wide application in industry as it can do without a large number of clamped diodes and capacitors, has no need for balanced capacitance and voltage, and has an easily modularized and expandable structure with good power quality.
However, an H-bridge multi-level inverter employed in actual industrial process contains a large number of H-bridges in each phase, which greatly increases the occurrence of open or short circuits for the switching devices. Further, with the increase of voltage, fault occurrence probability increases. An H-bridge multi-level inverter indeed provides convenience for applying electrical and electronic techniques in high voltage and large power applications, but once a fault takes place, a small one might cause factory shut down, while a severe one might result in catastrophic incidents and huge societal loss. Research indicates that switching device faults account for 82.5% of faults of the whole drive system in an inverter-powered variable frequency speed regulation system, and thus a switching device is the most vulnerable sector in the drive system.
Currently, there are two fault-tolerant strategies for countering inverter open circuit IGBT faults. One of the strategies is the hardware redundant method of adding redundant bridges or redundant modules. Such a method may operate with full load, but is at the cost of increase of cost, inverter weight, and complexity. In situations where volume and weight are strictly restricted, such a method is not adoptable. The other strategy taking reduction of manufacturing cost into account is to make use of the available switching devices and to operate under reduced load, wherein fault-tolerant objective is achieved by means of altering the control algorithm. Traditional multi-level inverter PWM waveform modulation algorithm is unable to adapt to inverter control subsequent to removal of fault modules, requiring a substitute thereof for fault controlling. The higher level of the inverter, the more pieces of redundant algorithm are required to be added in. Moreover, algorithm switching requires fault diagnosis and algorithm selection. In a high level multi-level inverter, fault types are numerous, time for overall algorithm selection is long, and thus system response time is extended.